Numerous types of mass spectrometer employ image current detection of ions Such spectrometers commonly employ Fourier transformation of the detected image current to produce the frequency and/or mass spectrum, hence giving rise to the name Fourier transform mass spectrometry (FTMS). Such mass spectrometers typically employ ion trapping devices, with which there is a need to control the ion population in the ion trap in order to limit space charge effects.
Clearly, it is desirable in FTMS to accumulate as many ions as possible in the mass analyser, in order to improve the statistics of the collected data. However, this is in conflict with the fact that there is saturation at higher ion concentrations caused by space charge effects. These space charge effects limit mass resolution and affect the mass accuracy, leading to incorrect assignment of masses and even intensities.
The total ion abundance accumulated within an ion trap may be controlled by automatic gain control (AGC) as described in detail in U.S. Pat. No. 5,107,109 and WO 2005/093782 for RF linear traps. First, ions are accumulated over a known time period and a rapid total ion abundance measurement is performed. Knowledge of the time period and the total ion abundance in the trap allows selection of an appropriate filling time for subsequent ion fills to create an optimum ion abundance in the trap.
There have been proposed a number of further ways to control ion population within the trap. For example, for RF ion traps as described in U.S. Pat. No. 5,572,022 and U.S. Pat. No. 6,600,154 it has been proposed to include a pre-scan just before the analytical scan in order to provide a feedback for automatically controlling the gating or fill time for introducing ions into the trap for the analytical scan. It has also been proposed to use an extrapolation of a multitude of pre-scans as in U.S. Pat. No. 5,559,325 for a similar purpose. In another method, disclosed in WO 03/019614, there has been proposed the use of an electrometer type detector of a triple quadrupole arrangement to measure the ion flux in transmission mode for determining fill times of subsequent analytical scans. In the case of FT-ICR, a method has been proposed in U.S. Pat. No. 6,555,814 which includes pre-trapping ions in an external accumulation device with subsequent detection on an electron multiplier.
In the case of FTMS, the instrument can be configured to use image current detection for determination of ion charge into the mass analyser. The ions are typically first trapped in an injection device, such as a linear trap, before transfer to the FT mass analyser and the ion current determined in the mass analyser can be used so that the ion number in the injection device is controlled to avoid space charge effects therein. For example, this approach is used in numerous Orbitrap™ electrostatic trap instruments from Thermo Fisher Scientific, in some cases along with automatic gain control (AGC) in the interfaced linear trap, where a short duration pre-scan (“AGC pre-scan”) is used for the estimation of ion currents.
It is desirable to improve the accuracy of ion number measurements in FTMS, especially using image current detection.